The scanning probe microscope (SPM) is a widely known type of apparatus for the surface observation or roughness measurement of metals, semiconductors, ceramics, synthetic resins and other materials. One representative of this type of apparatus is an atomic force microscope (AFM), which measures an interatomic force acting between a probe and the sample surface. The atomic force microscope has several measurement modes, among which a method called a “non-contact mode” or “dynamic mode” has been popularly used in recent years. In this method, a cantilever provided with a probe is vibrated at a frequency near its resonant point. In this state, the force that acts on the probe due to the interaction with a sample surface is converted to a change in the amplitude, phase or frequency of the vibration of the cantilever, and this change is detected.
FIG. 5 is a configuration diagram showing the main components of a conventionally known scanning probe microscope, which is disclosed in Patent Document 1 or other documents. A sample 1 to be observed is held on a sample stage 2 mounted on a substantially cylindrical scanner 3. The scanner 3 includes an X-Y scanner 3a for scanning the sample 1 in the two directions of X and Y axes perpendicular to each other and a Z scanner 3b for slowly moving the sample 1 in the Z-axis direction perpendicular to both X and Y axes. These scanners are each driven by a piezoelectric element which creates a displacement when a voltage is externally applied to it. A cantilever 4 with a probe 5 at its tip is provided above the sample 1. The cantilever 4 is vibrated by an oscillating unit including a piezoelectric element (not shown).
To detect the displacement of the cantilever 4 in the Z-axis direction, an optical displacement detection unit 10, which includes a laser source 11, two mirrors 13 and 14, and a photodetector 15, is provided above the cantilever 4. In the optical displacement detection unit 10, a laser beam emitted from the laser source 11 is reflected by the mirror 13 to a substantially perpendicular direction so that the laser beam is cast on the tip of the cantilever 4. The light reflected from the cantilever 4 is redirected to the photodetector 15 by the mirror 14. The photodetector 15 has a light-receiving surface, which is either divided into a plurality of sections (normally two) arranged in the displacement direction (Z-axis direction) of the cantilever 4, or divided into four sections arranged in the Z-axis and Y-axis directions. A vertical displacement of the cantilever 4 causes a change in the proportion of the light received by each of these light-receiving sections. Accordingly, the amount of the displacement of the cantilever 4 can be calculated by computationally processing the detection signals corresponding to the amounts of light received by those sections.
The non-contact mode measurement operation of the scanning probe microscope having the previously described configuration is briefly explained. The driving unit (not shown) vibrates the cantilever 4 in the Z-axis direction at a frequency near its resonance point. In this state, if an attracting or repelling force acts between the probe 5 and the surface of the sample 1, the vibration amplitude of the cantilever 4 changes. This slight change in the vibration amplitude is detected based on the detection signal of the photodetector 15, and a feedback control of the piezoelectric element of the Z scanner 3b is performed to move the sample 1 in the Z-axis direction so as to cancel the aforementioned change, i.e. so as to maintain the vibration amplitude at a constant value. In this state, when the piezoelectric element of the XY scanner 3a is controlled to scan the sample 1 in the X-Y plane, the amount of the aforementioned feedback control relating to the Z-axis direction will reflect micro-sized irregularities on the surface of the sample 1. By using a signal produced by this operation, a surface image of the sample 1 can be created.
In the scanning probe microscope having the previously described configuration, the emission angle (emission direction) of the laser beam emitted from the laser source 11 may change slightly due to a temperature-dependent change in the characteristics of the laser oscillation circuit or other factors. Such a change in the emission angle of the laser beam causes a problem as hereinafter described.
FIG. 6 schematically shows a normal operation of the optical displacement detection unit 10. When the probe 5 is scanning a flat region on the sample 1, the light reflected from the cantilever 4 forms a spot at a position P on the light-receiving surface of the photodetector 15, as shown in FIG. 6(a). When the probe 5 reaches a bulge 1a on the sample 1, the cantilever 4 is bent upward, as shown in FIG. 6(b), causing a downward displacement of the spot position P of the reflected light on the light-receiving surface of the photodetector 15. This displacement causes a change in the detection signal from the photodetector 15, from which one can obtain information reflecting the elevation of the bulge 1a or other properties.
On the other hand, FIG. 7 schematically shows an operation in the case where the direction of the laser beam emitted from the laser source 11 is inclined upward. As can be seen in FIG. 7(a), when there is no inclination (displacement) of the emission direction of the laser beam (which is normally emitted parallel to the X-axis in the present case), the light reflected from the cantilever 4 forms a spot at a point P on the light-receiving surface of the photodetector 15. This is the same as in the case of FIG. 6(a). In this state, for example, when the emission direction of the laser beam is slightly inclined upward due to a change in the ambient temperature, the incident angle of the illuminating light Lm to the cantilever 4 changes accordingly. Therefore, although the probe 5 is scanning the flat region of the sample 1, the spot position P of the reflected light on the light-receiving surface of the photodetector 15 is displaced downward. That is to say, the light-receiving surface of the photodetector 15 receives light in a manner similar to the case where the bulge 1a is present on the surface of the sample 1 as shown in FIG. 6(b). Thus, if the aforementioned change in the emission direction (emission angle) of the laser beam occurs, the system will falsely recognize it as a concave or convex portion on the surface of the sample 1.
One possible method for preventing such a false recognition is to use a laser source 11 whose emission angle does not significantly change with temperature or other factors. Another possibility is to provide a temperature controller for maintaining the ambient temperature of the laser source 11 at a constant level. However, any of these solutions cannot be implemented without a significant increase in cost. Furthermore, they do not work effectively against a change in the emission angle due to a non-temperature factor, such as an aging variation.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-233669